The present invention relates to an apparatus for driving an objective lens for reading out information from information tracks recorded spirally or concentrically on a disc-shaped record medium by projecting a light spot onto the record medium by means of the objective lens.
In an apparatus for reproducing or picking-up an information signal from the above mentioned record medium, the record medium is usually called as a video disc in which encoded video and audio signals are recorded as optical information such as an optical transmissivity, reflection, phase property. While the video disc is rotated at a high speed such as thirty revolutions per second, i.e. 1,800 rpm, a laser beam emitted from a laser light source such as a helium-neon gas laser is focussed on the tracks of the disc as a light spot and the optical information is read out. One of important properties of such a record medium is a very high density of recorded information and thus a width of the information track is very narrow and a space between successive tracks is also very narrow. In a typical video disc described in, for instance, Philips Technical Review, Vol. 33, 1973, No. 7, a pitch of the tracks amounts only to 2 .mu.m. Therefore the diameter of light spot should be correspondingly small such as 1 to 2 .mu.m. In order to pick-up correctly the recorded information from such tracks having very narrow width and pitch a mutual positional error between the light spot and tracks, i.e., a tracking error should be reduced as little as possible. In U.S. Pat. No. 3,909,608 and No. 3,882,317, it has been proposed to effect a so-called tracking control in which a mutual displacement of the light spot and track is photoelectrically detected to produce a tracking error signal and the light spot is moved or shifted in a direction perpendicular to the track in accordance with the detected tracking error signal. In the U.S. Pat. No. 3,882,317 as a tracking mechanism use is made of an oscillating mirror inserted in an optical path from a laser light source to an objective lens, and the mirror is rotated in accordance with the detected tracking error signal. However such a tracking mechanism could not attain a sufficiently high accuracy and is liable to be large in size. Further the known tracking mechanism is rather expensive.
In order to avoid the above mentioned drawback of the known tracking mechanism it has been further proposed that the objective lens on its holder is supported by a resiliently supporting member comprising springs, e.g. leaf springs, and the objective lens is moved in a direction perpendicular to the optical axis of objective lens as well as to the tangential direction of the information track by means of an electromechanical transducer in accordance with the tracking error signal. Hereinafter the direction in which the objective lens is moved for effecting the tracking control is termed as a tracking direction. As the electromechanical transducer use may be made of electromagnet, voice coil, piezoelectric element, etc. In order to obtain a good response property for tracking, the assembly should be made small in size and light in weight. In an actual reading-out apparatus in addition to the tracking error a so-called focussing error is produced in which the light spot could not be correctly focussed on the information track. For correcting the focussing error there should be provided with a focussing mechanism for moving or displacing the objective lens in a direction of its optical axis. When the tracking mechanism is installed on the focussing mechanism, the tracking mechanism should be smaller in size and lighter in weight for attaining an accurate focussing correction. In the tracking mechanism with the electromagnet a sufficiently large force necessary for moving the objective lens can be produced and the mechanism can be constructed small and light in a relatively simple manner. However, an accurate tracking correction could not be effected, because a relation between an amount of electric current passing through a coil of the electromagnet and an amount of displacement of the objective lens is not linear. In the tracking mechanism comprising a voice coil, a miniaturization could hardly be attained. Further the tracking mechanism with the piezoelectric elements could not produce a sufficently large driving force for driving the objective lens.
In order to avoid the above mentioned drawback, the applicant has proposed an apparatus for driving objective lens in U.S. patent application Ser. No. 139,756 filed on Apr. 14, 1980. FIGS. 1A and 1B are plane (half cross sectional) and longitudinal sectional views, respectively showing one embodiment of the apparatus for driving an objective lens 1 in a two-dimensional direction disclosed in the aforesaid U.S. patent application Ser. No. 139,756 filed on Apr. 14, 1980. A lens holder 2 made of a magnetic material is coupled with an intermediate cylindrical body 4 by means of a pair of leaf springs 3 and 3' which extend upright in parallel with each other symmetrically with respect to an optical axis of the lens 1. Moreover, a cylindrical body 17 made of magnetic material is coaxially arranged outside the lens holder 2. Therefore, the objective lens 1 can shift in a tracking direction shown by an arrow A perpendicular to an optical axis as shown in FIG. 1A. The intermediate cylindrical body 4 is secured to an outer cylindrical body 6 by means of a pair of spirally disc-shaped springs 5 and 5'. Therefore, the intermediate cylindrical body 4 is arranged movably in the optical axis of the objective lens 1 as shown by an arrow B in FIG. 1B. First and second yokes 7 and 7' cooperating with the lens holder 2 made of magnetic material and permanent magnets 8, 8' are secured to an outer cylindrical body 6 by a yoke supporting ring 15. In this manner, the lens holder 2 can be moved in the optical axis direction together with the intermediate cylindrical body 4 by supplying electric current to the coil 11. Since the yokes 7, 7' and magnets 8, 8' are fixedly arranged, the movable portion of the apparatus can be made light in weight. Coils 9 and 9' are would around the first and the second yokes 7, 7', respectively. Further, a moving coil 11 is wound around a ring 10 formed integrally with the cylindrical body 4 and a ring-shaped permanent magnet 12 and ring-shaped yokes 13, 14 cooperating with the coil 11 are secured to the outer cylindrical body 6 by means of a ring-shaped holder 16.
In the apparatus mentioned above, it is possible to move the objective lens 1 by a given amount in the direction A perpendicular to the optical axis by supplying an electric current corresponding to a tracking error to the first and second coils 9, 9'. Therefore, it is possible to obtain a sufficiently large force for effecting the tracking control, and to make the apparatus small and light in its size and weight. Moreover, it is also possible to move the objective lens 1 by a given amount in the optical axis direction B by supplying an electric current corresponding to a focussing error to the coil 11.
FIG. 2A is a characteristic curve of the above mentioned apparatus showing a relation between a driving frequency of focussing control and an amount of deviation of the lens in the optical axis. In FIG. 2A, a vertical axis shows an amount of deviation of the objective lens in the optical axis as dB, and a horizontal axis shows a driving frequency of the focussing control signal supplied to the moving coil 11. The characteristic curve includes undesired disturbance near 500 Hz marked by a due to resonance of arms of the disc-shaped springs 5, 5'. FIG. 2B is a characteristic curve of the tracking mechanism representing a relation between an amount of deviation of the lens in the tracking direction and a driving frequency of the tracking control signal supplied to the coils 9, 9'. This curve also includes disturbance in a range between 2 KHz and 10 KHz due to resonance of the leaf springs 3, 3' as shown by b in FIG. 2B. Therefore, the known apparatus for driving the objective lens has a serious drawback that focussing control and tracking control can not be performed accurately because of the disturbances in the characteristic curves due to the resonance of the resilient supporting springs.